Fluid pulsing device

ABSTRACT

A fluid pulsing device suitable for controlling the output of a chimney to enable the smoke to be discharged as smoke rings comprises an input passageway such as the chimney stack, and output passage such as a vent to atmosphere and a reservoir for smoke. Switching means switches the smoke alternately to the reservoir and to the output passageway, whereupon entrainment empties the reservoir. The switching means may be fluidic, mechanical or means for resonating fluid.

1451 Sept. 26, 1972 United StatesA Patent Baker et al.

[54] I- FLUID ULSING DEVICE then such an installation is often unsuccessful in dispersing the pollution.

It is known that the dispersal of waste products from a chimney is assisted if the charge is pulsed to form buoyant vortexrings known as smoke rings. The creation of the ring enables the hot gas to retain its heat and kinetic energy without diffusion into the atmosphere for longer than would otherwise be possible. As a result the charge remains lighter than the surrounding air for a longer period and rises higher into the atmosphere before dispersal.

According to the present invention there is provided i a fluid pulsing device comprising an input passageway,

anoutput passageway, a fluid reservoir, and a switching means for switching a fluid flow from the input passageway between a first flow path in which fluid flows from the input passageway to the reservoir, and a second flow path in which fluid flows from both the input passageway and the reservoir to the output passageway.

ln a preferred arrangement the passageways and reservoir are so shaped and so positioned that, in

operation', a fluid flow from the input passageway to the output passageway entrains fluid from the reservoir and a flow of fluid from the input passageway to the reservoir entrains fluid from the output passageway.

The'device may include static deflecting means so shaped and positioned as to be ycapable of directing a fluid flow from the input passageway substantially wholly to either the output passageway, or the reservoir.

It is further preferred that the output passageway extends for 'at least part of its length in substantially the same direction as the input passageway, and the reservoir comprises a third passageway which extends for at least part of its length in substantially the opposite direction to the first passageway.

The device may be such that the passageways and reservoir are coaxial with the input passageway and reservoir being positioned one within the other, and the deflecting means is positioned at least partly across the end of the input passageway and is shaped to deflect a fluid flow from the input passageway along a radial flow path to the output passageway or to the reservoir.

The switching means may comprise means for directing a control pulse or stream of fluid into one or more of the passageways and/or the reservoir.

The means for switching may comprise means for directing a control pulse or stream of fluid along the output passageway in a direction away from the junction with the input passageway in such a manner as to switch the said fluid flow from the input passageway to a flow path leading from the input passageway to the output passageway.

The switching means may include means for terminating a pulse of fluid passing from the output passageway, the terminating means comprising means for directing a control pulse or stream along the output passageway in a direction towards the junction with the input passageway.

The switching means may comprise means for temporarily increasing the flow of fluid through the input passageway in such a manner as to switch the fluid flow from the input passageway to a flow path leading from the input passageway to thel output passageway.

The switching means may comprise a fourth passageway opening into the output passageway or reservoir, and means for producing pressure waves in a body of fluid in the fourth passageway to provide periodic pressure variations for switching the fluid flow in the input passageway between the said two flow paths.

The switching means may include periodic timing means for switching a fluid flow in the input passage between the said two flow paths periodically.

The apparatus may be made to be bistable so that the main flow remains directed to either the input passageway or the reservoir after switching, or may be made monostable so that the main Aflow is stable in only one of the two flow paths.

It is an important advantage of the invention that the provision of a fluid reservoir reduces back pressure pulses on the source of the fluid. The reservoir allows a continuous input to be converted to a pulsed output.

The invention finds particular use in stable atmospheric conditions which have a temperature inversion, for example in a valley where the smoke from a continuous plume forms localized high concentrations. Vortex rings can be made to penetrate through such inversions, allowing dispersion and dilution over a much wider area.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. l is a longitudinal section along the axis of a fluid pulsing device embodying lthe invention for a chimney;

FIG. 2 is a longitudinal section of an alternative pulsing device embodying the invention;

FIG. 3 is a block diagram of a system for operating the device of FIG. l;

FIGS. 4a and 4b are diagrammatic representations of a flap valve for use in FIGS. l or 2;

FIG. 5 is a block diagram of an alternative system to that of FIG. 3; and

FIG. 6 is a longitudinal section of an alternative pulsing device embodying the invention with a block diagram of an operating system.

' Referring to FIG. l, there is shown a pulsing device for a chimney, comprising a housing indicated generally at l0. An input passageway ll is defined by a first cylinder 12 which may consist of or may be secured to the end of the main stack of the chimney. An output passageway 13 is defined by a second cylinder 14 arranged to pass smoke from the input passageway 1I to atmosphere through an outlet l5. A third passageway 16 is constituted by the annular space between the first cylinder l2 and a third cylinder I7 which surrounds and is coaxial with the first cylinder 12. The third cylinder 17 may lead to a smoke storage cavity or reservoir, or may be of sufficient width and length to constitute the reservoir itself. The cavity is vented to the atmosphere at a point remote from the top of the third cylinder 17.

Static deflecting means 18 are provided to allow deflection of smoke rising in the first passageway 11 to a selected one of the output and third passageways 13 and 16. The deflecting means 18.comprises an upper deflector member 18' having a substantially frusto conical shape coaxial with the cylinders 12, 14 and 17. A lower deflector member 19 is formed on the end of the first cylinder 12 and comprises a thickening of the ends of the cylinder so that in section either side of the lower deflecting member 19 has the form of a lobe.

A control pipe 20 at the axis of the first cylinder l2 rises through the upper deflector member 18' to a control nozzle 21 at the apex of the deflector member 18'. Application of a control jet through the nozzle 2l causes smoke rising in the input passageway 1l to pass radially out from the end of the passageway 1l round the lower part of the deflector member 18', and up to the output passageway 13.

A number of control nozzles 22 are positioned in a ring around the second cylinder 14 pointing downwards towards the deflector member 18' and supplied with air by a passageway not shown in FIG. l. Application of a control pulse of air to the nozzles 22 causes smoke rising in the input passageway 1l to pass radially outwards below the lower part of the deflector member 18' and thence downwards along the third passageway 16. Thus the nozzles 22 form a fluidic ring terminator. Alternatively, a single jet pipe terminator may be used. t

The apparatus can be made bistable or monostable. In the example shown, at low flow rates the apparatus is bistable and, once the appropriate control jet of air has been applied, the smoke flow will pass stably either to the output passageway 13, or down the third passageway 16. At high flow rates the device is monostable in the downward direction, but upward flow along the passageway 13 can be produced as long as the control jet of air is emitted from the control pipe 20.

In operation, at the start of a cycle, waste products rising in the input passageway l1 pass around the lower deflector member 19 down into the storage cavity from the third passageway 16. When a required charge of waste product exists in the storage cavity, a pulse of air is emitted from the control nozzle 21 which causes the smoke from the input passageway 11 to flow around the upper deflector member 18' into the output passageway 13. As soon as a required amount of charge from the storage cavity has passed from the third passageway 16 by entrainment up to the output passageway 13 and out of the outlet 15, pulses of air are applied to the control nozzles 22 to terminate the flow out of the outlet l5, so that the smoke rising from the input passageway l1 returns to its original path into the third passageway 16.

When the smoke from the input passageway l1 is passing vinto the third passageway 16, air is entrained from the output passageway 13 down into the third passageway 16. Conversely1 when the charge in the storage cavity is passing rapidly up from the third passageway 16 to the output passageway 13, smoke is entrained from the input passageway 11. These entrainments result in two advantages over known pulsing units for chimneys. The first entrainment in the downward direction contributes to a clean break to the end of the pulsed charge which forms the required smoke ring, and prevents dribbling of smoke over the edge of the outlet 15. The second entrainment in the upward direction reduces any possible back pressure applied down the first passageway ll to the boiler producing the smoke to be ejected.

A further advantage of the apparatus embodying the present invention and shown in FIGS. l and 2 arises in that no moving mechanical parts are necessary for the flow switching, avoiding the severe wear problems which may result from mechanically moving apparatus in the head of a chimney stack.

Referring to FIG. 2 there is shown an alternative embodiment of the invention which has many elements in common with those of FIG. l, and like elements will be referred to by like reference numerals.

In FIG. 2 the lower deflector member 19 is similarly positioned within the third cylinder 17 but the upper deflector member 18 comprises a lip which extends radially inwards from the end of the third cylinder 17. The output passageway 13 is formed by a second cylinder 14 consisting of an extension of the third cylinder 17 The control pipe 20 terminates just below the lower deflector member 19, and the apparatus is shown without control nozzles 22 of FIG. l, although these may be added if required. In the alternative arrangement of FIG. 2, the input passageway 1l is formed in the annular space between the first cylinder l2 and the third cylinder 17, and the third passageway 16 is formed within the first cylinder 12.

In operation the waste products from the boiler rise along the input passageway 11 and initially pass around the lower deflector member 19 down through the third passageway 16 to the storage cavity. When the required charge has been collected, a pulse of air is applied up the control pipe 20 which causes the charge from the storage cavity to flow up the third passageway 16 and out of the smoke outlet 15 and also causes upward entrainment of the smoke in the passageway 1l to flow around the upper deflector member 18' also to the outlet 1S. The arrangement shown can be made to be monostable so that, upon termination of the pulse of the nozzle 21, the smoke rising in the passageway I1 passes again down through the passageway 16 to the storage cavity.

As with the embodiment of FIG. l, a clean break to the end -of the smoke rings can be obtained by downwards entrainment while smoke is passing to the storage cavity, and upward entrainment during the discharge prevents back pressure on the boiler. In general however the arrangement of FIG. l is to be preferred, as this allows the storage cavity to be positioned conveniently around the outside of the top of the main chimney stack 12, whereas the arrangement of FIG. 2 requires the passage of smoke from the inner cylinder 12 to an outside storage cavity secured to the outside of the main chimney stack 17.

In FIG. 3 there is shown a block diagram of means for applying the required control signals to the pulsing means shown in FIG. l or to the pulsing means shown in FIG. 2. By way of example the block diagram is shown as operating the pulsing means of FIG. l.

The main chimney stack l1 receives hot waste gases at relatively low pressure from the boiler 26. An air compressor 27 passes air at high pressure to a high pressure air storage vessel 28 with a gauge 29. The air then passes through a tube flow meter 30 and pressure regulating valve 31 to a controlled pressure storage vessel 32 with a further gauge 33. The controlled pressure output of the vessel 32 is fed through control valves 34 and 35 to the control pipe 20 and the fluidic ring terminator 22 respectively. The air flows to these elements from the control valves 34 and 35 are controlled respectively by electrical solenoid valves 36 and 37. These valves 36 and 37 are controlled in turn by an electrical timing circuit 38 which opens the solenoid valves alternately for predetermined intervals of time to effect the switching sequences already described.

The ring terminator 22 has as its function the clean termination of the vortex rings.

As an alternative to the fluidic ring terminator 22 of FIGS. l and 3, there is shown in FIGS. 4a and 4b a mechanical ring terminator formed by a flap valve 39 positioned near the opening of the output passageway 13. Such a valve may be operated electrically by the timing circuit, or by way of further example, pneumatically by a pneumatic timing circuit.

In FIG. 5 there is shown a further embodiment of the invention which may use the pulsing device shown in FIGS. l and 2, and in which the switching means comprises means for resonating the pipe 20. An oscillator 39 operates an electronic relay 40 which in turn operates an electromechanical relay 4l which opens and closes the solenoid valve 36 at a frequency such as to produce resonant waves in the control pipe 20. The

waves in the pipe produce at the opening 21' of the pipe 20 alternating high pressure pulses and rarefactions which switch the main smoke flow in the input passageway 11 as has been described with reference to FIGS. l and 2. Such a device may also be operated by forced oscillations not necessarily in resonance with the pipe. As has been explained, the apparatus can be made bistable or monostable. In the example shown in FIGS. l and 2, at low flow rate the apparatus is bistable and, once the appropriate control pulse of air has been applied, the smoke flow will pass stably either to the output passageway 13, or down the third passageway 16. The embodiment of FIG. 5 is operated in such conditions, the alternating high pressure pulses and raritications in the pipe 20 at the opening 2l providing the required control pulses.

Referring now to FIG. 6 there is shown a further embodiment of the invention in which a mechanical valve is used not merely as a smoke ring tenninator as in FIGS. 4a and 4b, but is used as the main switching means for switching the fluid flow between the input and lthird passageways. In FIG. 6 elements correspond ing to like elements in previous figures are indicated by like reference numerals.

The input passageway 11 is formed within the third passageway 16 as in FIG. l, but the output passageway 13 is positioned directly opposite the mouth of the input passageway l1. The switching means comprises a flap valve 23 rotatable on a spindle 24, positioned transversely across the second passageway 13. The flap valve is rotatable between an open position in which the flap is parallel to the general direction of the passageway 13, and a closed position in which the flap is perpendicular to the general direction of the passageway 13.

The operation of the embodiment of FIG. 6 cor responds to that of the previous embodiments. The valve 24 is operated periodically by an electrical or fluidic timing circuit 24', and at the beginning of each cycle is closed. The waste gas then rises in the passageway ll and is deflected by the closed flap 23 to the reservoir passageway 16. When sufficient charge has collected in the reservoir, the flap valve 23 is opened and the waste gas from the passageway 1l passes direct to the passageway 13, and entrains further waste gas from the passageway 16, thus emptying the reservoir.

Although the embodiment of FIG. 3 suffers from the disadvantage of exposure of a mechanical valve to the waste gas, a number of advantages can be achieved.

The pressure of the hot gases from the boiler 26 can be increased by a fan 42 so that the waste gas itself provides the pressure flow to eject the vortex ring when the flap valve is opened. The separate compressed air supply used in the previous embodiments for ejecting the rings can be eliminated as the motive power for ejecting the rings is now provided by the smoke itself. A nozzle 25` can be fitted on to the cylinder l2 defining the input passageway 11, in order to raise the velocity of ring ejection sufficiently. The ratio of throat area to nozzle area can be made such that the ejector works at optimum efficiency.

The chimney diffuses out from the throat 25 to the larger diameter outlet l5 in order that the requirements for effective ring formation (volume, frequency, etc.) can be matched to the performance of the ejector.

The storage volume cross-sectional area of the reservoir increases towards the lower end in order to reduce the downward velocities, and to reduce the height of the storage vessel.

It will be appreciated that the embodiment of the invention suitable for any particular application, for example a particular chimney, will depend upon many factors concerning the position of the chimney, the nature of the effluent and the general operating conditions. A number of these factors will now be considered by way of example with reference to the embodiments described hereinbefore.

By way of example, it is found that good rings can be produced if the column of smoke within the chimney has come to rest between each pulse, the position of the pulse control pipe relative to the end of the chimney is about two stack diameters, the length to diameter ratio of the pipe is about 6 to I, and there is a sharp cutoff at the back of the pulse. If the rings are produced too frequently they can interfere and destroy one another.

As the fluid is ejected in the form of a slug it contacts the surrounding atmosphere and is deformed in a similar manner to'that occurring in the process of the formation of the head of a rivet. One of the limiting fac tors controlling the minimum time interval that must elapse between the generation of successive rings is that the ambient fluid should be sufficiently motionless for it to deform the second slug of ejected fluid as described above.

One of the more important parameters effecting ring formation is the exit velocity profile. lt is suggested that a higher wall shear velocity profile is preferred for good ring formation.

There are many methods for establishing a given profile on the chimney exit including;

a. Varying the length of the stack above the ejector b. Varying the size of the ejector pipe c. Fitting a honeycomb type of grid in the stack d. Changing the outlet shape e. Varying the roughness of the stack The effect of increasing the length of the stack above the ejector is that the gases have a greater opportunity to become less turbulent and the velocity profile to become more fully developed.

On the other hand increasing the stack length increases the pressure drop due to friction and also puts up the capital cost of the chimney.

Fitting honeycomb in the top of the stack has the the effect of dampening out the large turbulent eddies and establishing the fully developed profile in a much shorter length of pipe.

Good rings can be obtained using a length of mesh honeycomb fitted flush with the outlet of the stack. However, the presence of the honeycomb in the stack can reduce the ejection velocity for a given blowing pressure due to its pressure drop. In practice there may some blockage occurring within the passages of the honeycomb over a loi'ig period of time.

The overall size of the chimney complex can depend to a large extent on the size of the storage vessel. The minimum size-of this volume must be equal to the maximum volume of gas which can be discharged from the furnace or boiler between each pulse. In practice it should be made larger to allow at least for the tolerance on the control mechanism and also for variations in gas temperature.

It may be desirable for an annular section reservoir to increase in size as it approaches the open end. This can help to reduce the velocity of the gases thus reducing the energy required to switch the flow during the entrainment process.

When the device is operating satisfactorily in a balanced state the smoke level in the storage vessel oscillates up and down without spillover. Increasing the buoyancy of the gas also helps to slow the gases down and therefore raise the mean level of the oscillation.

It may be desirable on a full size chimney to install sensors within the storage vessel preferably at the top and bottom. If the If time is too long so that spillover occurs the bottom sensor can detect the onset of this and signal the control equipment to speed up. On the other hand if the cycle time is too fast for the flow rate of effluent into the stack, the top sensor within the vessel can detect that the smoke level has risen and signal the system to slow down. If the cycle time is allowed to continue at a faster rate than required, fresh air would eventually be entrained up from the'bottom of the vessel and finally, get blown out of the top within the rings.

The operation of the control system has been described with reference to FIGS. 3, 5, and 6. It is found that for a given ejector pressure there is usually a range of ejector pulse times, for which rings could be produced. Very short times result in slow rings with a small volume, and longer times result in faster rings containing a greater volume. The longer the pulse time the greater the speed to which the smoke is accelerated. The length of the smoke slug emitted from the chimney is also increased. Similarly an increase in the intermediate vessel pressure enables a reduction to be made in the ejector pulse time while the amount of smoke in each ring is maintained constant.

It is found that smoke is emitted from a chimney for a time which is greater than the pulse time. This means that the smoke is accelerated to a given velocity, and then continues to flow from the chimney with that velocity, (although some reduction is brought about by friction) until a terminator operates and brings the smoke column to rest. Immediate operation of a terminator after the end of the ejection pulse is not usually an efficient way of Observation large flow rates through the chimney. Observation suggests that once the rolling action of the ring is started, it is still possible to add more smoke to the ring without having to provide a continuous acceleration.

Whereas various ejector pulse times can be employed (each producing its own size of ring) it is important that the timing of the terminator and of the overall frequency is matched to the pulse time. If the terminator is closed too early, insufficient smoke is put into each ring and the excess finds its way into the atmosphere via the storage volumne. lf the If closes too late, an excess amount of smoke flows from the chimney outlet after the ring has been formed, and the ring may be followed by a short plume of smoke. Assuming that the terminator is operated at the correct time interval after the ejector pulse, thus forming a ring, it is still important that the frequency should be correct. Too low an operating frequency again results in excess smoke flowing from the storage vessel, and too high a frequency allows fresh air to be drawn into the chimney.

The mark-space ratio may be varied within the range 1:9 to 1:15. Very low mark-space ratios result in very low chimney flow rates since low ratios are achieved by reducing the frequency. Again, larger values of markspace are obtained primarily by raising the frequency since too long a pulse length causes too much smoke to be ejected.

As has been described a simple air operated butterfly valve may be used as a ring terminator in the stack as an alternative to an air terminator, and when placed about four stack diameters down from the top, the valve operates effectively.

The main advantage of the mechanical terminator is its shorter response time compared with the air valve. A mechanical valve can also be left closed until just before the next ring is ejected thus minimizing smoke leakage out of the stack.

The power consumption required to operate the mechanical valve is negligible compared with that vrequired to operate the air terminator.

If the air terminator operates correctly it can be shown that the energy needed to stop the smoke must be equal to the energy required to accelerate the smoke by the ejector minus the energy in the ring and other small losses in the stack.

We claim:

l. A fluid pulsing device comprising;

an input passageway;

an output passageway;

a fluid reservoir, said input passageway and said fluid reservoir being positioned one within the other; and switching means for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway. 2. A device according to claim l in which static deflecting means is positioned at least partly across the y end of the input passageway and is formed with a flow shape deflecting a fluid flow from the input passageway along a radial flow path to the output passageway, or the reservoir.

3. A device according to claim 1 in which the switching means comprises means for directing a control pulse or streaml of fluid along the output passageway in a direction away from the junction with the input passageway to switch the said fluid flow from the input passageway to a flow path leading from the input passageway to the output passageway.

4. A device according to claim l in which th switching means includes means for terminating a pulse of fluid passing from the output passageway, the terminating means comprising means for directing a control pulse or stream along the output passageway in a direction towards the junction with the input passageway.

5. A device according to claim l in which said switching means comprises a fourth passageway opening into the output passageway, or reservoir, and means for producing pressure waves in a body of fluid in the fourth passageway to provide periodic pressure variations for switching the fluid flow in the input passageway between the said two flow paths.

6. A device according to claim l in which said input passageway, said output passageway, and said fluid reservoir are substantially coaxial.

7. A fluid pulsing device comprising;

an input passageway;

an output passageway;

a fluid reservoir;

static deflecting means positioned at least partly across the end of said input passageway and formed with a flow shape deflecting a fluid flow from said input passageway along a radial flow path to said output passageway or to said reservoir;

and switching means for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway.

8. A fluid pulsing device comprising;

an input passageway;

an output passageway;

a fluid reservoir;

and valve means in said output passageway for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway.

9. A fluid pulsing device for a chimney comprising;

a chimney structure for the passage of hot gas; j combustion means producing hot, waste gas, said combustion means feeding said hot waste gas to said chimney structure;

an output chimney vent for venting the hot waste gas to atmosphere;

a reservoir for said hot waste gas;

and switching means for switching the flow of hot waste gas from the chimney structure between a first flow path in which the hot waste gas flows from the chimney structure to the reservoir, and a second flow path in which the hot waste gas flows from both the chimney structure and the reservoir to the output chimney vent.

l0. A device according to claim 9 in which the switching means includes periodic timing means for switching a fluid flow in the chimney conduit between the said two flow paths periodically.

ll. A device according to claim 9 in which the output passageway terminates in an opening so shaped that fluid ejected therefrom produces a vortex ring. 

1. A fluid pulsing device comprising; an input passageway; an output passageway; a fluid reservoir, said input passageway and said fluid reservoir being positioned one within the other; and switching means for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway.
 2. A device aCcording to claim 1 in which static deflecting means is positioned at least partly across the end of the input passageway and is formed with a flow shape deflecting a fluid flow from the input passageway along a radial flow path to the output passageway, or the reservoir.
 3. A device according to claim 1 in which the switching means comprises means for directing a control pulse or stream of fluid along the output passageway in a direction away from the junction with the input passageway to switch the said fluid flow from the input passageway to a flow path leading from the input passageway to the output passageway.
 4. A device according to claim 1 in which the switching means includes means for terminating a pulse of fluid passing from the output passageway, the terminating means comprising means for directing a control pulse or stream along the output passageway in a direction towards the junction with the input passageway.
 5. A device according to claim 1 in which said switching means comprises a fourth passageway opening into the output passageway, or reservoir, and means for producing pressure waves in a body of fluid in the fourth passageway to provide periodic pressure variations for switching the fluid flow in the input passageway between the said two flow paths.
 6. A device according to claim 1 in which said input passageway, said output passageway, and said fluid reservoir are substantially coaxial.
 7. A fluid pulsing device comprising; an input passageway; an output passageway; a fluid reservoir; static deflecting means positioned at least partly across the end of said input passageway and formed with a flow shape deflecting a fluid flow from said input passageway along a radial flow path to said output passageway or to said reservoir; and switching means for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway.
 8. A fluid pulsing device comprising; an input passageway; an output passageway; a fluid reservoir; and valve means in said output passageway for switching a fluid flow from said input passageway between a first flow path in which fluid flows from said input passageway to said reservoir, and a second flow path in which fluid flows from both said input passageway and said reservoir to said output passageway.
 9. A fluid pulsing device for a chimney comprising; a chimney structure for the passage of hot gas; combustion means producing hot, waste gas, said combustion means feeding said hot waste gas to said chimney structure; an output chimney vent for venting the hot waste gas to atmosphere; a reservoir for said hot waste gas; and switching means for switching the flow of hot waste gas from the chimney structure between a first flow path in which the hot waste gas flows from the chimney structure to the reservoir, and a second flow path in which the hot waste gas flows from both the chimney structure and the reservoir to the output chimney vent.
 10. A device according to claim 9 in which the switching means includes periodic timing means for switching a fluid flow in the chimney conduit between the said two flow paths periodically.
 11. A device according to claim 9 in which the output passageway terminates in an opening so shaped that fluid ejected therefrom produces a vortex ring. 